The instant invention relates to a process for manufacturing integrated circuits comprising, on the one hand, insulated gate MOS transistors provided with one gate level and, on the other hand, components provided with at least two gate levels, integrated on the same semiconductor substrate.
FIGS. 1A to 1F show the conventional manufacturing steps of an integrated circuit comprising, on the one hand, a MOS transistor on the left part of the figure and, on the other hand, a double gate component, for example a memory.
FIG. 1A shows an initial step of the process wherein, on a semiconductor substrate 1, for example a silicon substrate, one has formed, on the one hand, a thin layer of silicon oxide (hereinafter called oxide for the sake of simplicity) 2 and, on the other hand, a layer of thick oxide 3 designed to insulate two components one from the other.
During the step illustrated in FIG. 1B, one has formed above the structure a first polycrystalline silicon layer 4, a second layer of thin oxide (or another electrical insulator) 5, a second layer of polycrystalline silicon 6 coated with a resist mask 7, the structure being represented after photoetching of the resist mask 7 and of the second layer of the polycrystalline silicon 6 for delineating the upper gate of the memory type component and the FET gate.
During the step illustrated in FIG. 1C, one uses as a mask the openings previously formed for etching the oxide layer 5 and leaving in place a portion of the oxide layer 8 between the two gates of the memory type component and a portion of the oxide layer 9 under the MOS transistor gate. Besides, it will be noted that the oxide layer 9 remaining on the MOS transistor may result from a portion of the layer 2 formed during the step shown in FIG. 1A or from a portion of the layer 5 formed during the step shown in FIG. 1B, depending on whether the portion on which one forms the MOS transistor has been masked or not during either of the oxidation steps.
During the step illustrated in FIG. 1D, the photoetching of the first level of polycrystalline silicon 4 is shown, the etchings previously carried out being used as a mask. It will be noted that during this etching phase one etches, on the one hand, as it is deemed desirable, the polycrystalline silicon 4 and, on the other hand, the silicon substrate apparent on the MOS transistor whereby recesses 10 are formed in this substrate on both sides of the gate area of this MOS transistor.
Then, as shown in FIG. 1E, the oxide layer 2 is etched out. It will be noted that, during this etching phase, one overetches the silicon layers 8 and 9 previously etched under the polycrystalline silicon areas 6. In FIG. 1E, the upper resist layer 7, which besides could have been removed during the step represented in FIG. 1D according to some technologies, is no longer shown.
After the step illustrated in FIG. 1E, doping steps are generally carried out, usually by means of ionic implantation, for forming the source and drain regions of the components. Before or after this step, a thermal oxidation step will also be carried out for laterally insulating the gates.
FIG. 1F shows an enlarged although schematic view of the gate area of the MOS transistor after a reoxidation phase. Due to the presence of overetchings in the oxide layer 9, during the reoxidation phase, a dilatation of the oxide layer will occur as well as a raising of the gate edges. The same phenomenon occurs for the upper gate layer 6 of the memory type component. This raising phenomenon of the gate area presents a quite specific drawback when it relates to a MOS transistor because the threshold voltages are therefore not precisely determined due to the fact that improperly determined voltages are applied on the limits of the transistor channel area. The drawback is less important as regards the upper gate of the memory type component since the transfer phenomena which are produced between said gate and the underlying gate are not specifically correlated with the lateral fields.
Another drawback of the process according to FIG. 1E is due to the fact that, during the drain and source doping steps of the MOS transistor, because of the presence of the recess 10 in the drain and source areas, the resulting doping profile will not be precisely controlled at the limits of the channel area.
Thus, an object of the instant invention is to provide for a process for manufacturing an integrated circuit simultaneously comprising MOS transistors and multi-gate components avoiding the drawbacks of the prior art such as hereinabove described.
In order to achieve this purpose, the instant invention provides for a process for manufacturing integrated circuits comprising insulated gate MOS transistors and components provided with at least a double gate component on a semiconductor substrate, comprising the steps consisting in insulating by means of insulating regions the areas wherein the various components will be formed forming a first insulating layer and a first gate level on the areas where the multi-gate components will be formed ; forming on the transistor areas and the multi-gate areas a second insulating layer, at least a second gate level and a first level of photoresist ; etching the first layer of photoresist and the second gate level according to the configurations chosen in the transistor areas and in the multi-gate ; areas coating the transistor areas with a second layer of photoresist ; selectively etching the second layer of photoresist in the center of the places where the drains and sources of the transistors are to be formed ; etching the apparent oxide areas, then the apparent gate and substrate areas ; removing the second layer of photoresist ; and carrying out an ionic drain and source implantation in one single step, possibly preceded by a thermal step.
An advantage obtained by this process is that there is no longer any raising phenomenon of the gate such as hereinabove described more particularly in connection with FIG. 1F and, moreover, as it will be seen hereinafter, a profile particularly interesting for the gate and drain areas is obtained in one single implantation step, this profile being usually designated by LDD (low drain diffusion), to indicate there is a shallower doping area at the channel limits and a deeper and more highly doped area at the places where the drain and source contacts will be established.
Another advantage of this manufacturing process is, during the doping step, that the diffusion depth is limited to the boundaries of the field oxide areas, which permits to obtain MOS transistors operating at a higher voltage.